Irish voters go to the polls on Thursday in the most fractious presidential election campaign since the republic was founded, with controversy dogging a number of the candidates.

Sean Gallagher, the TV star businessman and, until recently, frontrunner in the race, will learn later if allegations that he was a "bag man" for the Fianna Fáil party have ruined his chances of winning.

During the last televised debate on Monday on RTE's Frontline programme, Martin McGuinness effectively ambushed Gallagher over an allegation that he had solicited €5,000 for a fundraising dinner.

Before Monday night's revelation, Gallagher, the star of the Irish version of Dragon's Den, was leading the race with 40% support, according to the last opinion poll.

He has described the allegations that he took the cheque from Hugh Morgan, the owner of Morgan Fuels and a convicted petrol smuggler, as "a political assassination attempt by Martin McGuinness and Sinn Féin".

McGuinness has spent most of the campaign fielding questions about his past as the IRA's chief of staff during the Northern Ireland Troubles. During the campaign trail he was criticised by relatives of Irish security force members shot dead by the IRA in the 1980s.

He has also encountered hostility in some sections of the southern Irish media. On the eve of the poll, Ireland's main evening paper, the Evening Herald, ran a page-long editorial saying: "Why no sensible Irish person should even think about voting for McGuinness." It was accompanied by a cartoon of the Sinn Féin MP and former deputy first minister of Northern Ireland with a Kalashnikov rifle in one hand, an Irish tricolour in the other and a satanic tail sticking out of his rear end.

The final controversy of the campaign concerning Gallagher and his poor television performance in response to the claims has shortened the bookmakers' odds on his main rival, Michael Higgins, winning the contest. The veteran Labour party candidate's odds were slashed in the last 48 hours to 8-13. A final online opinion poll conducted by the website ballotbox.ie found that Higgins was in the lead among the seven contenders with 41%.

As well as electing their head of state – a purely symbolic post that holds no executive power – Irish voters will also be asked to vote on two referendums. The first is non-controversial, with the electorate expected to endorse the Fine Gael-Labour coalition's plans to dramatically cut judges' pay. The government was obliged by the constitution to get popular endorsement for the pay cut.

The second referendum has been far more contentious, with the government seeking extra powers to allow parliamentary committees to investigate all areas of Irish public institutions. The Irish Council for Civil Liberties has urged people to vote no in the latter referendum, which it believes will give politicians too much power to encroach in the lives of ordinary citizens and pose a threat in particular to a free media.